In oil wells which utilize electrical submersible pumps, it is common practice to employ low-pressure wellheads rated under 3,000 psi. They rely upon rubber packing glands to effect seals around the production tubing and the power cable. Where a seal is used between production tubing of a given diameter and power cable of different diameter to seal against the inside of the wellhead body, rubber glands are not effective against fluid pressures within the wellhead.
Nevertheless, these relatively low-pressure wellheads are popular in the industry because they are adequate when used with electrical submersible pumps; they are relatively inexpensive; and few metal parts are needed. Furthermore, the assembly is relatively easily installed because the production tubing and the electrical power cable can and do come directly out of the well without being mechanically sealed or secured to the wellhead.
Although, as noted, wellheads of the type continue to be quite widely used, there are certain problems which arise. One of these problems arises from the use of rubber, when it is relied upon for long-term sealing. The actual procedure for achieving the seal involves the squeezing of the rubber of the gland to force it against different diameters of tubing and cable. With such geometry, effective seals to the wellhead body, and against both tubing and cable, are difficult to achieve. In fact, maintenance of a low-pressure seal of about 500 psi is about the limit of effectiveness. Other problems also are encountered from the use of rubber for long-term protection. Gas is always present in oil wells, and rubber is particularly vulnerable to the gas which attacks the underside of the rubber and ultimately results in gas leakage as the rubber breaks down. The top side of the rubber, being exposed to the weather, is also subject to breakdown followed by gas leakage. The gas problem is aggravated further by the drilling of adjacent new wells and by the employment of water flooding to increase oil recovery. In the case of adjacent new drilling, high-pressure gas may be released through the formation and creates a safety hazard, and water flooding leads to an increased concentration of hydrogen sulfide gas from which the rubber packing is particularly susceptible to damage. Finally, the electrical cable is usually sealed only on its outer surface to the wellhead and gas may continuously escape through the wellhead by passing inside the cable jacket. Such passage of gas also occurs through the interior of electrical connectors now in use. These generally consist of an assembly of parts about the ends of the cable being used to carry power to the submersible pump. Gas under high pressure from the wellhead tends to force its way through the body of the electrical connector and presents a serious hazard in the form of an explosive atmosphere about the wellhead.
The present invention is concerned with the solution of the problems outlined. The primary object of the invention is a seal which not only withstands full pressure, but which also renders the wellhead environmentally safe. A further object of the present invention is a seal which meets all standards of the National Electric Code. A still further object of the invention is a seal in which all critical elements are made of tight-fitting metal parts resulting in long life and the absence of gas leakage. Yet another object is an explosion-proof connector which is intrinsically safe electrically.